1. Field of the Invention
The present invention relates to an electric cooking hob and to a method for determining the location of cooking utensils on the cooking hob. More particularly, the present invention relates to a cooking hob having a plurality of thermal cells distributed in matrix formation below a heat-resistant surface on which cooking utensils can be located in random manner.
Cooking hobs having devices for sensing pot position (and for simultaneously energizing the related heating elements below the pot) are known in the art of cooking appliances, such class of cooktops being called “High versatility cooktops”. These cooktops allow the user to place a cooking utensil in any part of the cooking surface, without being compelled to position the utensils in predetermined fixed positions. High versatility cooktops are usually realized by dividing the cooking area into small heating elements usually arranged into hexagonal or orthogonal grids.
Despite having been disclosed long time ago, these cooktops never reached the market due to a huge complexity of the proposed technical implementation. It is an object of the present invention to disclose some method to reach an industrially feasible implementation, by solving a number of issues present in the technical solutions according to prior art.
2. Description of the Related Art
In order to be convenient, such high versatility cooktops should include some systems able to deliver heat only below the pot location, in order to energize only the part of the cooktop actually covered by the cooking utensil(s). Such systems may rely on mechanical switches, thermal load identification or optical techniques. All of these techniques are, in practice, hardly feasible because all of them make use of a large number of discrete sensors, each one having to work at extremely high temperatures usually reached inside the heaters (up to 1000° C.). The technical solution disclosed in EP-A-1206164 in the name of the present applicant describes a technique that addresses the latter problem by using the heating elements themselves as cooking utensil sensors. Such method works by injecting into each one of the heating cell an alternating current, radio-frequency (RF-AC) signal and detecting the induced signal in one or more conductive loops placed between the cooking utensil and the heating cell, such induced signal being substantially changed by the pot presence. This known solution also discloses one possible electrical method to apply both the power current needed to heat-up the elements and the RF-AC signal needed to sense the presence of pots. The suggested method, despite being meritorious, has the disadvantage that the pan detection and power currents cannot be applied exactly at the same time but they need to be non-overlapping in time. This means that the action of detecting the presence of cooking utensils on a given thermal cell matrix (each thermal cell being a single small heating electrical resistor) requires the complete switch off of the power for a time that, in practice, cannot be lower than some tenth of milliseconds. The temporary switch off of the load can rise problems in the compliance with the “flicker” norms imposed in most industrialized countries.
It is therefore an object of the present invention to solve the problem of the simultaneous application of both the power current and RF-AC current to the heating cells of a matrix organized high versatility cooktop.